1. Field of the Invention
The present invention relates to a motor control device and, more particularly, to control of a synchronous motor according to a DQ transformation.
2. Description of the Related Art
There is known a method of DQ transforming three-phase currents into two phase currents of a d-phase and a q-phase, and then controlling each phase. In a control method of controlling a synchronous motor based on orthogonal DQ coordinate axes, a change in a maximum output voltage of a motor control device due to a change in a power supply voltage is assumed. In this case, parameters having a voltage margin are conventionally set so as to be able to stably drive a synchronous motor. A reactive current or a current control phase is set so that the torque becomes the largest in the lowest conceivable input voltage state.
FIG. 10 is a characteristic diagram showing a relationship between a reactive current and a rotation speed and a relationship between a continuous output torque and a rotation speed, according to a conventional reactive current control. According to the conventional reactive current control, a reactive current that is required for rotation at the assumed minimum voltage must be supplied. A continuous output torque decreases rapidly in a high-speed area.
FIG. 11 and FIG. 12 are characteristic diagrams showing a relationship between the phase advance and a rotation speed and a relationship between instantaneous maximum torque and a rotation speed, according to conventional phase advancement. FIG. 11 shows an example of a phase advance control that is suitable when the voltage is low. The phase advance is controlled constantly regardless of voltage. Therefore, the torque decreases when the voltage is high. On the other hand, FIG. 12 shows an example of a phase advance control that is suitable when a voltage is high. Because the phase advance is controlled constantly regardless of the voltage, the torque decreases when the voltage is low.
When a power supply voltage changes, a maximum voltage that a motor control device can apply to a motor changes due to this change in the power supply voltage. Therefore, when a current command is generated based on parameters that is set when the power supply voltage is maximum, the voltage becomes short when the power supply voltage decreases to a minimum value, and the current control becomes unstable. In this state, a voltage command value inside a controller becomes large due to a reduction in the power supply voltage in order to maintain a terminal voltage of the motor that rotates at the same commanded rotation speed. By utilizing this characteristic, a motor control device is proposed as disclosed in Japanese Patent Application Unexamined Publication No. 2003-52199. This motor control device changes a base speed (starting speed) at which a d-phase current is started to be passed based on voltage command values that are output as functions of d-phase and q-phase current command values in a high-speed rotation area. Based on this, a maximum current in the d-phase and a minimum current in the d-phase are adjusted to indirectly adjust a d-phase current command value, thereby decreasing the influence of a variation in the power supply voltage.
In the case of setting a reactive current or a current control phase so that the torque becomes the largest in a conceivable minimum voltage state due to a variation in the power supply voltage, a d-phase current of more than necessary is supplied because a voltage margin of the motor control device is taken into account. As a result, the motor output efficiency relative to current becomes low, and motor heating increases.
The motor control device described in Japanese Patent Application Unexamined Publication No. 2003-52199 uses characteristics of the inside of the controller to change a base rotation speed at which a d-phase current is started to be passed based on voltage commands that are output as functions of the d-phase and q-phase current commands in the high-speed rotation area. With this arrangement, the motor control device indirectly changes the d-phase current command to decrease the influence of variation in the power supply voltage. This motor control device does not directly control the reactive current or directly control the current phase according to a change in the input power supply voltage.